MKB wrote:IMO what you are paying Fulltone for is the ears and knowledge to combine such tiny mojo effects together to make a superior whole.

(I'm not picking on you MKB, I swear... )

...But I hear this stuff a lot and have to respond:

This sounds real good when you say it fast, but when I see stuff like a 4.7M shunt resistor hanging off of a 500k output volume pot wiper, it appears that whoever "designed" the circuit had absolutely no clue what he was doing...

DougH wrote:Here's a couple frequency plots of the "OCD" output stage in "HP" and "LP" mode to compare. I ran an AC simulation in SIMETRIX, swept from 10Hz to 20KHz, 3v peak input signal (with a 4.5v DC bias) into the output stage, fixed the tone control pot at 5k, left out the 56k to simplify, etc... R3 was changed from 33k for "HP" mode to 13.2k for "LP" mode to compare.

jg wrote:3V peak? meaning 6 V p-p? Into a circuit with a gain of ~3.8 powered by 9V? Hmmmmm.

Yeah John, I just pulled the voltage level out of my butt... I didn't take the 1st stg into acct just to keep it simple. I figured it *might* be able to do 6v p-p, and that might be a better representation of the signal (instead of 1v) if the gain was turned up.

I can do it again at 1v but I don't think the basic freq response will change.

I would have also expected shelving of the higher frequencies since there is no cap directly to ground but your sim doesn't show that......

There is somewhat of a shelf from 1k to 4k. I didn't duplicate all the fixed resistor details in the tone control and etc because it wasn't necessary to show the basic effect of the series resistance changes due to the switch.

I can sim the circuit from input to output if I get some time, if that would help. But I don't think the basic story of what the series resistance does will change.

I've had plenty of coffee, thanks...

I just get bored talking about the same ol' same ol'... Which means I probably should do something else.

Most pspice simulators don't care about levels when sweeping for gain, which to me already casts a shadow of doubt on their accuracy especically when you are driving things into the non-linear region. Pspice is a linear simulator. But using levels that are obviously too high for the circuit will always leave you questioning. IMO.

The shelving would probably show up if you run the sim up to 100K. The opamp starts rolling off around 4.8 kHz, which is probably the second rolloff you are seeing. The pot value you have set makes the max attenuation of around 17.8 dB LP(or 11.2 dB HP) and you see the effect the tone control has is around that before the rolloff of the opamp takes over. When the opamp is done rolling off it will have unity gain so the output would be 17.8 dB below that, your graph doesn't go that low for the one trace and doesn't go down to -11.2 dB for the other.

From your simulation result I would say the values look like they have been rather well chosen such that the opamp rolloff takes over just as the tone control would start to shelve. Between the gain setting R and C on the inverting input, the 220pF feedback cap, the tone control and the HP/LP switch, I'd say all these things have been very well 'centered' even though it gives the appearance of being very simple.

The upper set of curves is "LP" mode while the lower set is for "HP". Tone resistor settings are 1k, 5k, and 10k.

I realize Pspice has its limitations, but it can be useful for comparing before/after effects relative to ea other. I'm not trying to prove/disprove how "well-centered" or well voiced the circuit is. Just trying to show the effect of the LP/HP resistors.

After all these curves my point still stands. The "HP/LP" switch is just a variable low-pass filter. Nothing more- nothing less. Doesn't affect low freq response, etc, etc... In this case, pspice agrees with my original intuition anyway.

The upper set of curves is "LP" mode while the lower set is for "HP". Tone resistor settings are 1k, 5k, and 10k.

I realize Pspice has its limitations, but it can be useful for comparing before/after effects relative to ea other. I'm not trying to prove/disprove how "well-centered" or well voiced the circuit is. Just trying to show the effect of the LP/HP resistors.

After all these curves my point still stands. The "HP/LP" switch is just a variable low-pass filter. Nothing more- nothing less. Doesn't affect low freq response, etc, etc... In this case, pspice agrees with my original intuition anyway.

I thought the HP/LP stood for High Peak/Low Peak (of the tone control)? Did you think it stood for High Pass/Low Pass or something?

jg wrote:I thought the HP/LP stood for High Peak/Low Peak (of the tone control)? Did you think it stood for High Pass/Low Pass or something?

Hey, I have no idea what it stood for.

Others in this thread mentioned a high pass filter effect.

But even if it does stand for high/low peak, it appears to be labeled backwards (according to the posted schem). I don't really care about the labeling though- wasn't addressing that.

On the website http://www.fulltone.com/ocd.asp it describes the switch settings as "Low Peak" and "High Peak" in the descriptions of the sound clips. So maybe there was some confusion about what they meant by a number of people.

It'a a bit strange that Fuller labled the switch high peak and low peak. Soundwise it should be labled more bass/less bass or
normal/bass cut, 'cos the HP setting cuts lows, nothing more.
The switch settings define two different rollof frequencies for the high pass filter (output cap + vol pot).

It's probably a perception thing. More high freq response can make something sound "thinner" even if the bass response is the same. I've found that my hearing perception tends to respond to the shape of the curve moreso than the absolute amplitude of a particular frequency band.